The invention relates to a higher strength steel, in the dimensional range of small section and wire product lines, which, as final product, is intended preferably for use in concrete constructions and as a feed stock for drawing shops.
Alloying and technological possibilities for increasing the strength of reinforcing steels are limited primarily by the requirement that the physical properties be such, that these steels are thermally stable and can be welded extensively. Accordingly, conventionally produced, hard-rolled reinforcing steels correspond at most to Strength Class III, provided that there has been no additional microalloying.
At the carbon equivalent for steels that have not been micro alloyed and are suitable for welding, the production of reinforcing steels of Strength Class IV requires the use of additional process steps such as cold work-hardening or thermal work-hardening of the steel. Thermally work-hardened reinforcing steel of Strength Class IV with partial martensite transformation in a perpheral layer is still suitable universally for welding. The corresponding cold work-hardened reinforcing steel, on the other hand, only permits WP welding, since relaxation processes set in already at relatively low temperatures in cold work-hardened steels.
The known thermally work-hardened reinforcing steels have a ferritic-pearlitic core. Their peripheral layer comprises a high-strength martensitic layer of greater but lesser thermal stability than the core. The further processing steps of the reinforcing steel, moveover, such as bending and thread rolling, affect the strength of the peripheral layer. During the manufacture of such steels in wire rolling mills, problems with laying the coils naturally arise as a result of the work-hardened surface layer and the therewith associated greater stiffness of the wire.
A drawing shop feed stock, similar to the thermally work hardened reinforcing steels, is also known. In contrast to the conventional patenting, the drawing shop feed stock has a corresponding heat-treatment structure in the peripheral layer of its cross section (DE-C 23 45 738). However, to guaranty the drawability of the wire, the heat-treatment structure in the peripheral layer must not exceed 33% of the total cross section.
The invention is directed to enlarging the material- and technology-related boundaries that limit the increase in strength of higher-strength reinforcing steels and to improving the processability of reinforcing steel and the drawability of drawing stock feed stock with a heterogeneous structural constitution or to make the manufacture of such drawing stock in small-section steel mills possible.